The present invention relates generally to an apparatus and method for transmitting an optical signal. More particularly, the present invention relates to an apparatus and method for reducing the total attenuation and non-linear effects of a long distance optical communication system.
In today""s worldwide communication systems, it is often necessary to extend a transmission line over a long distance, which may include a body of water, to provide a communication link between a transmitter and a receiver. The current trend in communication systems is to use optical fibers to make these transmission lines. Optical fibers are preferred because the fibers can transmit a large number of digital signals at a high data transmission rate.
To further improve the signal carrying capacity of the transmission line, optical fibers can be used with Wavelength Division Multiplexing (WDM) technology. This technology allows multiple optical signals to be sent through the same fiber at closely spaced wavelength channels. This greatly enhances the information carrying capacity of the overall transmission system.
Several problems are encountered when optical fibers are used to transmit signals over a significant distance. For example, the power of the optical signal decreases as the signal travels through each fiber. This power loss, also called attenuation, can be compensated for by including amplifiers along the transmission line to boost the power of the signal. The placement and number of amplifiers along the transmission line is partly determined by the attenuation of the optical fiber. Obviously, a signal sent through a fiber with a low attenuation needs fewer amplifiers than a signal sent over a fiber with a high attenuation.
Chromatic dispersion is another problem encountered when transmitting signals over optical fibers. Chromatic dispersion, hereafter referred to as xe2x80x9cdispersion,xe2x80x9d arises from the optical fiber transmitting the different spectral components of an optical pulse at different speeds, which can lead to the spreading or broadening of an optical pulse as it travels down the transmission line. Each optical fiber has a dispersion value that varies as a function of the wavelength of the optical signal and arises from the material composition of the glass optical fiber and the waveguide characteristics. The dispersion within the optical fiber at a given wavelength can be positive, negative, or zero, depending on the transmission characteristics of the fiber. Despite the type of dispersion (positive or negative), excessive amounts can lead to detection errors at the receiver of the optical signal.
Transmitting signals at the zero-dispersion wavelength of a fiber will practically eliminate the dispersion problem, but can exacerbate other transmission problems, particularly non-linear effects when used with WDM systems. A particularly relevant non-linear effect in WDM systems is the phenomenon of Four Wave Mixing (FWM). FWM occurs when at least two signals verifying phase matching conditions are sent through the same fiber (as in WDM systems) and interact to generate new wavelengths. In the case of WDM systems having a large number (more than two) of equally spaced channels, these new wavelengths will eventually overlap with the signal wavelengths, thus degrading the Signal-to-Noise Ratio. It is known that WDM systems that have an operating wavelength different from the zero-dispersion wavelength of the transmission fiber (and therefore have a non-zero dispersion value at the operating wavelength) minimize FWM degradation. More precisely, FWM efficiency xcex7, defined as the ratio of the FWM power to the per channel output power (assuming equal input power for all the channels) is approximately proportional to:   η  ∝            [                                    n            2                    ⁢          α                                      A            eff                    ⁢                                    D              ⁡                              (                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  λ                                )                                      2                              ]        2  
where xcex1 is the fiber attenuation; n2 is the nonlinear refractive index; Aeff is the fiber effective area; D is the dispersion; and xcex94xcex is the channel spacing. The above approximation is valid under the condition xcex1 less than  less than xcex94xcex2, where xcex94xcex2=(2xcfx80c/xcex2)xc2x7Dxc2x7xcex94xcex2, c is the speed of light and xcex the transmission wavelength. See D. W. Peckham, A. F. Judy and R. B. Kummer, ECOC ""98, paper TuA06, pp. 139-140. As can be seen, for a given set of values for xcex94xcex, n2, and xcex1, to decrease FWM efficiency one can increase the absolute value of dispersion and/or increase the value of fiber effective area Aeff. On the other hand, decreasing channel spacing dramatically increases FWM efficiency.
Other non-linear effects include Self Phase Modulation, Cross Phase Modulation, Stimulated Brillouin Scattering (SBS), and Raman Scattering (SRS). It is well known that a fiber with a larger effective area at the operating wavelength is less susceptible to all non-linear effects.
To solve the dispersion and non-linear effects associated with sending signals through long optical fibers, conventional systems use transmission lines that connect spans of optical fiber that have alternating dispersion values. For example, a span of negative dispersion fiber can be followed with a span of positive dispersion fiber to even out the overall dispersion over the transmission line. This approach ensures that the dispersion is non-zero at local values throughout the transmission line to avoid non-linear effects and that the total dispersion over the cumulative transmission line is compensated to nearly zero at the receiver.
Various publications discuss different approaches to solve these problems. For example, U.S. Pat. No. 4,969,710 to Tick et al. discusses an optical fiber transmission path wherein total dispersion of the system is compensated by the use of fibers composed of glasses with total dispersion of opposite signs at the operating wavelength for the system.
U.S. Pat. No. 5,343,322 to Pirio et al. discusses a system for long distance transmission of a digital signal. The system uses optical fiber having a low negative dispersion to connect receiver stations that include dispersion compensation devices having positive dispersions to compensate for the negative dispersion.
U.S. Pat. No. 5,559,920 to Chraplyvy et al. discusses an optical communication system having an initial span of a strong negative dispersion followed by positive dispersion spans. The system overcompensates for the negative dispersion in that the final dispersion value is not zero.
Other publications, such as U.S. Pat. No. 5,587,830 to Chraplyvy et al., U.S. Pat. No. 5,719,696 to Chraplyvy et al., U.S. Pat. No. 5,675,429 to Henmi et al., and U.S. Pat. No. 5,778,128 to Wildeman also discuss transmission lines for long range systems. These publications disclose transmission lines that use varying combinations of fiber that have either a negative dispersion or a positive dispersion at the operating wavelength. The negative dispersion fiber and the positive dispersion fiber are arranged so that the total dispersion of the system is compensated to approximately zero.
Similarly, U.K Patent No. 2 268 018 also discusses an optical transmission system that combines optical fiber having a negative dispersion with fiber having positive dispersion to compensate the dispersion to zero for the total length of the transmission.
European Patent Application No. 0 790 510 A2 discusses a symmetric, dispersion-managed fiber optic cable. The cable of this disclosure includes a conventional single mode fiber having a positive dispersion at the operating wavelength connected to a second optical fiber that has a negative dispersion at the operating wavelength.
Applicants have noted that these prior arrangements use combinations of optical fiber that result in undesirably high levels of attenuation. Moreover, Applicants have noted that the optical fiber used in conventional systems does not adequately provide performance for reducing non-linear effects while minimizing attenuation.
In general, the present invention involves an optical transmission system and method for transmitting optical signals over a significant distance. In particular, the invention involves an apparatus and method for reducing the attenuation and non-linear effects of the optical transmission system.
In accordance with the purpose of the invention as embodied and broadly described herein, the invention is directed to an optical transmission line that includes first and second spans of single-mode fiber. The fiber of the first span has a negative dispersion with an absolute value of between about 2.5 ps/nm/km and 10 ps/nm/km at the operating wavelength. The second span is connected to the first span and has a positive dispersion at the operating wavelength. The positive dispersion of the second span compensates for the negative dispersion of the first span such that the cumulative dispersion across the first and second spans is approximately zero.
Preferably the absolute value of the negative dispersion of the first span at the operating wavelength is between about 3 ps/nm/km and 8 ps/nm/km, more preferably between about 4 ps/nm/km and 7 ps/nm/km.
Preferably the fiber of the first span has a zero dispersion wavelength of between about 1600 nm and 1670 nm and the operating wavelength is approximately 1560 nm.
In an embodiment the positive dispersion of the second span is between about 10 ps/nm/km and 20 ps/nm/km at the operating wavelength. Preferably a atio of the length of the first span to the length of the second span is less than about 7:1, more preferably less than about 5:1.
In another embodiment the fiber of the second span is a half-dispersion-shifted fiber having a zero dispersion wavelength between about 1350 nm and 1450 nm. In this embodiment the positive dispersion of the second span is preferably between about 7.5 ps/nm/km and 15.5 ps/nm/km at the operating wavelength, more preferably between about 8 ps/nm/km and 13 ps/nm/km, and even more preferably between about 9 ps/nm/km and 12 ps/nm/km, and/or a ratio of the length of the first span to the length of the second span is less than about 6:1, preferably less than about 4:1. Preferably the half-dispersion-shifted fiber has an attenuation equal to or less than about 0.195 dB/km at the operating wavelength.
In another aspect, the invention is directed to a high-speed optical communications system having an operating wavelength. The high-speed communications system includes a transmission line having first and second spans. Each of the first and second spans has at least one single-mode fiber. The fiber of the first span has a negative dispersion with an absolute value of between about 2.5 ps/nm/km and 10 ps/nm/km at the operating wavelength. The fiber of the second span has a positive dispersion at the operating wavelength. The positive dispersion of the second span compensates for the negative dispersion of the first span such that the cumulative dispersion across the first and second spans is approximately zero. There is also provided a transmitting device coupled to one end of the transmission line and a receiving device coupled to the other end. The transmitting device sends a signal across the transmission line to the receiving device.
Preferably the absolute value of the negative dispersion of the first span is between about 3 and 8 ps/nm/km, more preferably between 4 and 7 ps/nm/km.
Preferably the fiber of the first span has a zero dispersion wavelength of between about 1600 nm and 1670 nm, and the operating wavelength is approximately 1560 nm.
According to an embodiment, the positive dispersion of the second span is between about 10 ps/nm/km and 20 ps/nm/km at the operating wavelength. Preferably a ratio of the length of the first span to the length of the second span is less than about 7:1, more preferably less than about 5:1.
According to another embodiment, the fiber of the second span is a half-dispersion-shifted fiber having a zero dispersion wavelength between about 1350 nm and 1450 nm. In this embodiment, the positive dispersion of the second span is preferably between about 7.5 ps/nm/km and 15.5 ps/nm/km at the operating wavelength, more preferably between about 8 ps/nm/km and 13 ps/nm/km and even more preferably between about 9 ps/nm/km and 12 ps/nm/km, and/or a ratio of the length of the first span to the length of the second span is less than about 6:1, preferably less than about 4:1. The half-dispersion-shifted fiber has an attenuation lower than about 0.210 dB/km at the operating wavelength, preferably lower than about 0.205 dB/km. Even more preferably the half-dispersion-shifted fiber has an attenuation equal to or less than about 0.195 dB/km at the operating wavelength.
In still another aspect, the invention is directed to a method of transmitting a signal over a transmission line. The method includes the step of adding the signal to the transmission line. The signal is transmitted over a first span of single mode optical fiber that has a negative dispersion with an absolute value of between about 2.5 ps/nm/km and 10 ps/nm/km. The signal is then guided down a second span of single mode optical fiber that has a positive dispersion to compensate for the negative dispersion of the first span. The second span of fiber compensates the total dispersion over the transmission line to approximately zero. The signal is then dropped from the transmission line.
Advantageously the signal is added to the transmission line with a transmitting device. Advantageously the signal is dropped from the transmission line with a receiving device.
Preferably the absolute value of the negative dispersion of the fiber of the first span is between about 3 ps/nm/km and 8 ps/nm/km, more preferably between about 4 ps/nm/km and 7 ps/nm/km.
In an embodiment, the fiber of the second span is a half-dispersion-shifted fiber having a zero dispersion wavelength between about 1350 nm and 1450 nm, and/or the ratio of the length of the first span to the second span is less than about 6:1.
In yet another aspect, the invention is directed to a single mode optical transmission fiber. The fiber includes a core and a cladding; the core comprises: an inner core that has a first refractive-index difference. A first glass layer surrounds the inner core and has a second refractive-index difference. The fiber has a peak refractive index difference less than or equal to about 0.0140, a zero-dispersion wavelength of less than about 1450 nm, a dispersion value of between about 7.5 and 15.5 ps/nm/km at an operating wavelength of about 1560 nm, and an effective area of at least 60 xcexcm2. The cabled fiber has a cutoff wavelength of less than about 1500 nm.
Advantageously, the peak refractive index difference of the fiber is less than or equal to about 0.0120 and, preferably, the core of the fiber is free from negative refractive index difference layers.
The fiber has an attenuation lower than about 0.210 dB/km at a wavelength of 1560 nm, preferably lower than about 0.205 dB/km and, even more preferably, equal to or lower than about 0.195 dB/km.
In a first embodiment of the fiber the first refractive-index difference is about zero and the second refractive-index difference is greater than zero. Preferably, the second refractive-index difference is about 0.0120.
In a second embodiment the fiber comprises a second glass layer surrounding the first glass layer and having a third refractive-index difference.
In a first version of the second embodiment the second refractive-index difference is greater than the first refractive-index difference and the third refractive-index difference, and each of the first, second, and third refractive-index differences are greater than zero. Preferably, the first refractive-index difference is about 0.0025, the second refractive-index difference is about 0.0070, and the third refractive-index difference is about 0.0017.
In a second version of the second embodiment of the fiber, first refractive-index difference is greater than zero, the second refractive-index difference is about zero, and the third refractive-index difference is greater than zero. The first refractive-index difference can be about 0.0097, in combination with a third refractive-index difference of about 0.0122. Preferably, however, the first refractive-index difference is between about 0.0070 and 0.0120 and the third refractive-index difference is between about 0.0030 and 0.0080.
In yet another aspect, the invention is directed to a high negative dispersion single-mode optical transmission fiber. The fiber includes a core and a cladding; the core comprises: an inner core having a first refractive index difference between about 0.0100 and 0.0160; a first glass layer surrounding the inner core and having a substantially constant refractive index difference, the first refractive index difference of the inner core being greater than the second refractive index difference of the first glass layer. Furthermore, the fiber comprises a second glass layer surrounding the first glass layer and having a third refractive index difference between about 0.0030 and 0.0080. The fiber, when cabled, has a cutoff wavelength less than about 1500 nm. The fiber has a value of dispersion between about xe2x88x928.0 ps/nm/km and xe2x88x923.0 ps/nm/km at a wavelength of about 1560 nm. Preferably, the inner core extends to an outer radius of between about 1.9 and 3.0 xcexcm, and the first glass layer extends from the outer radius of the inner core to an outer radius of between about 3.5 and 8.0 xcexcm and the second glass layer has a width of between about 1.5 and 4.0 xcexcm.
Throughout the present description reference is made to refractive index profiles of optical fibers. The refractive index profiles comprise various radially arranged sections. Reference is made in the present description to precise geometrical shapes for these sections, such as step, alpha-profile, parabola. As is well known to one of ordinary skill in the art, fiber manufacturing process may introduce changes in the shape of the structural sections of the described, idealized, refractive index profiles, such as a central dip in the proximity of the fiber axis and diffusion tails associated with the refractive index peaks. It has been shown in the literature, however, that these differences do not change the fiber characteristics if they are kept under control.
In general, a refractive index profile section has an associated effective refractive index profile section which is different in shape. An effective refractive index profile section may be substituted, for its associated refractive index profile section without altering the overall waveguide performance. For example, see xe2x80x9cSingle Mode Fiber Opticsxe2x80x9d, Luc B. Jeunhomme, Marcel Dekker Inc., 1990, page 32, section 1.3.2 or U.S. Pat. No. 4,406,518 (Hitachi). It will be understood that disclosing and claiming a particular refractive index profile shape, includes the associated equivalents, in the disclosure and claims.
Moreover, it is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.